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Full text of "Text Book Of Mechanical Engineering"

Numerical Examples.

553

EC
H. P. given out by generator = 7   =

H. P. Turbine must give to dynamo =

R of circuit, taking lead and return
(dia. of wire = "192, area = "03,
7=2640 ft.)

 -n   i         -         -

H.P. lost in wire =

746

300 x 50

746

20

"86

2640 x 8*4
03 x 1,000,000

jjo_xjo x 74

746 ~

= 20

= 23-25

74 ohms

2-48

H. P. delivered to motor

is   that   generated   less

=      20-2*48         = I7'52

that lost in wire.
H. P. available at shop shafting =   17*52 x '84
But H. P. given by turbine was 23*25

1471

          ~ .            H. P. taken out     14*71

Gross efficiency = -----   -.  = 

---------------------        H.P.put in        23*25

= '6327    or 63! %

and efficiency of circuit only 

H.P. delivered to motor
H. P. generated

= 2? = -876   oi*87*6/0
there being 12*4% of the generated H. P. lost in the wire.

Two actual cases may be quoted (i) 4^ H. P. was trans-
mitted 8 miles through %" telegraph wire, with a total efficiency
of 30 %. (2) The dynamos having a resistance of 470 ohms, and
the circuit 950 ohms, the line being 34 miles long, a total effi-
ciency of 32 % was obtained by decreasing revolutions from 2100
at generator to 1400 at motor, the potentials dropping 2400 to
1600 volts, a method of working first advised by Siemens.

(Seep. 929.)

Storage cells are objectionable for tramcar and locomotive
driving on account of their great weight, 2 tons of cells being
about the weight required for a i-ton car. The following results
are from an actual experiment with Faure accumulators :

35 cells of 95 Ibs. each
H. P. absorbed in charging
Time of charging
Lost work in charging
Chemically stored energy
Recovered electric energy

tons.

2 2 hrs. 45 mins.

34%

66%

60 % of 66 % = ,